Conventional structural assemblies and engine components, as used in the manufacture of military and commercial aircraft and missiles, are commonly formed of lightweight, high strength materials such as aluminum alloys and titanium alloys. These assemblies can be formed using manufacturing methods such as die-forging and investment casting. Although die-forging and investment casting allow for high material utilization rates by producing near-net shape members, these methods can be prohibitive due to high nonrecurring tooling costs and long lead times for both tool design and manufacturing, especially where only a limited number of aircraft will be produced. An alternative manufacturing method used to form aircraft structural assemblies is hogout machining from plate or hand forgings. However, this method is also prohibitive, as it can result in a low material utilization rate in addition to increased machining time.
In seeking better manufacturing methods to reduce both the cost of manufacture and the weight of the structural assemblies and engine components, as well as to increase performance, the use of metal matrix composites has been proposed. Metal matrix composites consist of one or more layers or stratums of reinforcing fibers in a metal matrix. One method of constructing metal matrix composites is to alternate layers of metal foil and reinforcing fibers with the first and last layers consisting of metal foil. To consolidate layers of foil and fiber, the foil/fiber pack is canned, evacuated, and subjected to a hot isostatic pressing (HIP) process. During the HIP process, the stratified materials are placed within a HIP furnace and heated to approximately 1,700.degree. Fahrenheit with an ambient pressure of 15,000 psi. Over time (2-4 hrs.), the metal foil becomes soft and flows between the fibers thereby consolidating the metal matrix. The same process is used to diffusion bond the metal matrix composite to the exterior surface of a structural member.
As shown in FIG. 1, during the HIP process, steel tooling 2 must be placed around the periphery of the structural member 4 and the metal matrix composite 6 to secure the composite to the member and to contain the metal matrix once it becomes soft and begins to flow. To prevent contamination of the metal, the steel tooling is welded together to form a sealed tool and the air inside the tool is evacuated prior to placing the tool in the HIP furnace. After the HIP process is completed, the steel tooling is removed using conventional machining methods. A residual layer of steel tooling typically remains around the periphery of the structural member, however, as complete removal of the steel tooling by machining may damage the metal matrix diffusion bonded to the surface of the tooling. Thus, the remaining steel tooling must be removed by chemical milling.
Metal matrix composites may also be formed using a manufacturing process known as tape casting. During tape casting, a metal powder, such as a titanium powder, is mixed with an adhesive or binder to form a slurry. The slurry is then spread over a fiber mat using a doctor blade to form a single-ply composite laminate. The composite laminate is then placed in a furnace and heated to 1,200.degree. Fahrenheit, a temperature well below the melting point of the titanium, but sufficiently high to burn off the binder. To consolidate one or more composite laminates and/or diffusion bond the composite laminate to the exterior surface of a structural member, the composite laminate must undergo a HIP process, including the construction of steel tooling.
Metal matrix composites may also be constructed by spraying molten metal onto the reinforcing fiber. For example, a fiber mat can be secured to a metal drum which is covered with titanium foil. The drum is placed in a vacuum chamber and rotated at a predetermined angular velocity. Molten titanium, formed by melting titanium powder at a temperature of approximately 2,700.degree. Fahrenheit, is sprayed onto the fiber mat using a plasma spray gun to form a single-ply composite laminate. As with tape casting, to consolidate one or more composite laminates and/or diffusion bond the composite laminate to the exterior surface of a structural member, the composite laminate must undergo a HIP process, including the construction of steel tooling.
In general, metal matrix composites are lightweight and provide high strength and stiffness at room and elevated temperatures. These composites are, however, expensive to produce due to the material costs of, for example, powder or foil titanium, and the high processing temperatures, press times, and expensive steel tooling of the HIP process which is necessary to consolidate the metal matrix and to diffusion bond the metal matrix composite to a structural member. As such, the use of these composites in end products also remains cost prohibitive.
Thus, there remains a need for manufacturing processes capable of producing lightweight, high strength structural assemblies and engine components for aircraft and missiles using metal matrix composites. These manufacturing processes must be cost effective in terms of both material and manufacturing costs to enable use of these innovative composites in end products.